1. Field of the Invention
The invention relates to a method for manufacturing an image-forming element having a hollow cylindrical drum body with a metallic outer layer and provided on its outer circumferential surface with a plurality of circumferentially extending electrodes which are electrically insulated from one another and from the drum body.
2. Description of Background Art
An image-forming element of this type is usable in a so-called direct induction printer the functional principle of which is described for example in EP-A1-0 247 699. In such a printer, the electrodes on the surface of the drum body are covered by a dielectric layer, and a rotatable sleeve is disposed along the drum body so that the surfaces of the drum body and the sleeve form a gap which extends at right angles to the electrodes of the drum. A stationary magnetic knife is disposed inside of the sleeve for generating a magnetic field in the gap. A uniform layer of electrically conductive and magnetically attractable toner powder is applied to the surface of the sleeve.
In an image-forming zone defined by the magnetic field in the gap, the toner powder is transferred onto the surface of the drum, depending on the voltage applied to the electrodes thereof. Thus, by rotating the drum body and energizing the electrodes in accordance with image information supplied to the control unit, a toner image is formed on the surface of the drum. Alternatively, a uniform layer of toner powder may be applied to the surface of the drum, and the toner powder may be selectively removed from the drum in accordance with the energizing pattern of the electrodes.
A conventional image-forming element and a method of manufacturing the same are disclosed in EP-A1-0 595 388. The electronic components of the control unit and a pattern of electric conductors are provided on a plate-like substrate. The conductors to be connected to the electrodes of the drum terminate at a rectilinear edge of the substrate, so that a terminal array is formed. The substrate carrying the conductor pattern and the electronic components is mounted inside of an aluminum cylinder forming the drum body such that the terminal array is inserted through a longitudinal slot of the drum body. The remaining free spaces in the slot are filled with epoxy resin so that the terminals are insulated from the drum body. The edge portion of the substrate projecting out of the slot is etched away so that only the ends of conductors are left, which will then slightly project beyond the surface of the cylindrical drum. The surface of the cylinder is then covered with an insulating layer (epoxy) having a thickness equal to the length of the projecting ends of the conductors. Then, the electrodes are formed by cutting grooves into the insulating layer and filling them with conductive material. Thus, each electrode will be in contact with the end of one of the conductors of the control unit. Finally, the electrodes are covered with a layer of dielectric material.
It will be understood that the pitch of the electrodes determines the resolution of the printer. For example, in the case of a printer with a resolution of 23.6 pixel per mm (600 dpi), the pitch of the electrodes will be no larger than approximately 40 .mu.m. Since a sufficient insulating gap must be provided between adjacent electrodes, the width of each individual electrode will be as small as approximately 20 .mu.m.
With the conventional manufacturing method, it is difficult and cumbersome to reliably and reproducibly form the electrodes, which may be several thousands in number, at the correct positions and with sufficient electrical insulation therebetween. EP-A1-0 595 388, cited above, mentions the possibility of replacing the insulating epoxy layer, into which the grooves are cut, with an oxide layer formed by anodizing the aluminum cylinder. Then, however, it would be even more difficult to cut the grooves into the comparatively hard metal oxide layer.